Chainsaws are traditionally made with guide bars, where the saw chain runs with the drivelink tangs in a groove along the perimeter of the guide bar and is carried at the front end of the guide bar by a toothed sprocket inserted in the guide bar. The guide bars are either solid, i.e., made from a single steel plate with a milled or ground groove, or laminated, i.e., made from three thinner plates joined by spot welding, with the groove being created by making the middle plate smaller than the side plates. The guide bars are usually hardened along the edges to improve the wear resistance where the chain slides along the guide bar, and unhardened or only slightly hardened between the edges.
It has been shown, however, that guide bars of the known types when used in vehicle-born tree harvester machines are easily damaged if the grip of the machine around the tree trunk is not firm enough. A common type of damage is whew the guide bar is bent near the end attached to the machine. It is often difficult or impossible to straighten such a guide bar, because of cracks in the hardened edge and fractures near the spot welds, where hard brittle regions border on soft heat affected zones.
U.S. Pat. No. 5,052,109 discloses that soft annealing of a zone across the width of the guide bar near the end attached to the machine will concentrate any bending to this zone, where the hardness both at the weld spots and at the previously hardened edges is low enough to avoid fracture, and that such a guide bar is easy to straighten after it has been bent. Disadvantages with this method are that the force a guide bar can withstand without bending is lower, and that the edges may become wrinkled during the concentrated bending and difficult to get smooth when the rest of the bar is straightened.
U.S. Pat. No. 4,965,934 discloses how the weld spots of a laminated guide bar can be made ductile without annealing or with a low temperature annealing that does not diminish the hardness imparted to the side plates during previous hardening. Disadvantages with this method are that some risk of cracks at the edges remains and that the lower middle plate hardness lowers the stiffness of the bar.
The present invention concerns a guide bar where the risk of cracks at the edges is eliminated without lowering of the bending resistance, and where any bending will not be so concentrated that the edges could get wrinkled.